There are no spare parts for these systems, so if your stop key is missing it may be difficult to replace.

Either you need to find someone selling a broken CX5M, or someone that has a broken system for parts that is willing to donate or sell you the stop key.

Or you could try to see if 3D printing one is an option, assuming you don't need any other parts like springs. For this you would first need to make a 3D model based on one of the existing keys, as I don't think this was done before for the CX5m.

Nowadays it's possible to manufacture your own keys. But as the procedure is complicated and smelly, doing this only makes sense if there's no way to buy a replacement key. I made a pair of keys for myself:

The model is also suitable for making the Stop key, some small changes are necessary. The model is here: